Routing devices within a network, often referred to as routers, maintain tables of routing information that describe available routes through the network. Network routers maintain routing information that describes available routes through the network. Upon receiving a packet, a router examines information within the packet and forwards the packet in accordance with the routing information. In order to maintain an accurate representation of the network, routers exchange routing information in accordance with one or more routing protocols, such as an interior gateway protocol (IGP) or Border Gateway Protocol (BGP).
The term “link” is often used to refer to the connection between two devices on a network. The link may be a physical connection such as a copper wire, a coaxial cable, any of a host of different fiber optic lines or a wireless connection. In addition, network devices may define “virtual” or “logical” links, and map the virtual links to the physical links. In other words, the use of virtual links provides a degree of abstraction.
Traffic engineering may be applied within a network for a variety of purposes, such as to route traffic around network failures or congested links or to direct certain traffic along a particular path through the network that meets a set of explicit requirements. For example, a router within the network may establish a label switched path (LSP) in a Multi-Protocol Label Switching (MPLS) network using a resource reservation protocol, such as the Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE). Once a packet is mapped on to an LSP by a label edge router (LER), the intermediate devices along the LSP forward the packet based on labels attached to the packet, rather than making independent forwarding decisions based on the packet destination and the intermediate devices' routing information. A Traffic Engineering MPLS LSP (TE LSP) may in this way be used to define and implement a path from a source device to a destination device that satisfies requirements for certain traffic transported by the network.
The explicit requirements that must be satisfied by an LSP represent constraints upon the set of possible paths from the source device to the destination device. These constraints, such as available bandwidth, direct shortest path first algorithms to compute a satisfactory path with regard to the constraint metrics. The network routers then establish an LSP that matches the computed path and, using the LSP, forward traffic in a manner that satisfies the constraints. Constrained shortest path first (CSPF) thus represents a fundamental building block for traffic engineering systems, including MPLS and Generalized MPLS (GMPLS) networks. However, constraint-based path computation in large, multi-domain, multi-region, and/or multi-layer networks is complex and may in some instances require cooperation between elements in different administrative domains that do not exchange sufficient traffic engineering information for computing multi-domain paths.
Network operators may augment the functionality of their networks by introducing one or more path computation elements (PCEs) that allow the network routers to offload path computation. A PCE establishes PCE communication protocol (PCEP) sessions with one or more path computation clients (PCCs) through a network. Path computation clients, such as routers, issue path computation requests to the PCE using their respective PCEP sessions. The PCE applies constraints provided in a path computation request to compute the path of a TE LSP through a path computation domain that satisfies the constraints. The PCE then returns the path to the requesting PCC, effectively augmenting the network path computation functionality. A PCE may be stateless or stateful. In general, a stateless PCE does not maintain state describing TE LSPs in the network. A stateful PCE, on the other hand, maintains state for a subset of TE LSPs in the network, allowing the stateful PCE to utilize more sophisticated LSP path computation algorithms in some instances.